In the event of a nuclear accident or a radiological attack, the exposure to ionizing radiation can cause acute damage to radiosensitive tissues that have rapid turnover rates, including the hematopoietic (H) system and gastrointestinal (GI) tract. After irradiation, an insufficient regeneration of either the hematopoietic system and/or the GI tract can lead to death within a few weeks, which is termed the acute radiation syndrome (ARS). Although significant progress has been made to understand mechanisms underlying the ARS, no FDA approved therapy is available to treat both the H-ARS and the GI-ARS when given at least 24 hours after irradiation. At this time point, the majority of tissue stem/progenitor cells will already be dead. Therefore, there is an urgent need to develop novel medical countermeasures (MCMs) that target master regulators of tissue regeneration in response to radiation injury. The long-term goal of this project is to develop a novel class of MCMs that mitigate both the H-ARS and GI-ARS by targeting calcium/calmodulin-dependent protein kinase kinase 2 (CaMKK2). We have shown that inhibition of CaMKK2 by genetic deletion or by the small molecule inhibitor STO-609 is sufficient to stimulate hematopoietic regeneration and mitigate the H-ARS. Remarkably, our preliminary data indicate that deletion of Camkk2 specifically in myeloid cells is sufficient to facilitate blood cell formation following total body irradiation. More recently, we found that deletion of Camkk2 also protected mice from the GI-ARS. Of note, outside the brain, the expression of CaMKK2 is restricted to a small number of cell types, including macrophages and epithelial tuft cells, which share the ability of tuning the regeneration rate of hematopoietic and intestinal stem cells. Based on these findings, we hypothesize that CaMKK2 is an important druggable target to regulate the behavior of hematopoietic and intestinal stem cell niches, and blocking this enzyme 24 hours after irradiation will be sufficient to facilitate tissue regeneration in response to radiation injury. We will test this hypothesis using sophisticated mouse models, CaMKK2 inhibitor, along with primary bone marrow cells and intestinal organoids from human donors. Using these combined approaches, we will define mechanism(s) by which CaMKK2 expressed in myeloid cells mitigate H-ARS, and will determine the impact of acute Camkk2 loss in tuft cells after irradiation on the development of GI-ARS. By completing this grant, we expect to gain new insight into the role of CaMKK2 in regulating both the H-ARS and GI-ARS. More importantly, by more comprehensively understanding mechanism(s) underlying the effects of CaMKK2 inhibition on mitigating the ARS, we will lay the foundation for approval of STO-609 as a medical countermeasure against radiation under the FDA?s Animal Rule.